Controllers for operating and governing the operation of actuators which in turn are coupled to a varsity of valves and other devices are known. Such controllers typically comprise one or more ports connectable to a pressurized fluid source, which by sequential control signals close and open pressure ports and venting ports thereof to thereby impart motion to various valves and the like, articulated thereto.
A number of differing designs have been formulated for actuator controllers, such as those utilizing dual electromagnetic actuators to move a valve spool in opposite directions. Another example is the use of a double wound actuator, able to energize in both directions.
An alternative approach is the so called spring return pneumatic actuators which typically comprise one or more cylinders slidingly accommodating therein a spring-biased piston, wherein the piston is spring biased in one direction and pneumatically urged in the opposed direction. Such actuators are at times referred to as single action pneumatic actuators. Accordingly, when compressed air is applied at one end of the piston, the piston is thrust to load the biasing spring so as to provide a useful output bias thrust. However, upon discharging the compressed air the piston is retuned and the spring member is relaxed, with a useful but reversed output linearly reducing thrust, as the spring relaxes. This arrangement offers strong spring-biasing effect at the initial displacement of the piston, whereby the final thrust available as the piston comes to rest is considerably less than the initial return thrust.
An example of such as design is discussed in GB Patent No. 1373070 to Tugwell disclosing a pneumatic actuator comprising a double-acting piston separating two first chambers, spring means to urge the piston in one direction, and valve means adapted to admit compressed air to one of the chambers and thus to load the spring means, then to transfer some of such air into the other chamber at a selected phase position of the piston, and then to open the said one chamber to atmosphere whereby the combined forces of the spring means and of the compressed air acting on the piston in the other chamber, complete the power stroke of the actuator.
Hereinafter in the specification and claims, reference will be made to a pressurized fluid useful for operating the controller, with particular reference to pneumatic devices operated by pressurized air. The skilled person will appreciate that such apparatuses are operable with either pressurized air or liquid, the former often being more readily available and suitable for industrial environments.